This invention relates in general to ultrasonic flowmeters and more particularly to circuitry for precise determinination of the differences in transit time of ultrasonic wave propagation upstream and downstream within a fluid flowing in a conduit.
One method of determining the velocity of flow of fluids within a conduit utilizing ultrasonic wave transducers has been to place a pair of transducers apart in the direction of flow and to determine the difference in propagation time of ultrasonic waves between these transducers in the upstream direction and the downstream direction. The flow velocity of the fluid, of course, adds to the sound velocity in the downstream direction and substracts from it in the upstream direction. One effective technique for accomplishing this measurement is known as the singaround method. In this method an ultrasonic wave is initiated at one of the transducers and is transmitted to the other transducer, for example, in the upstream direction. Upon receiving this transmitted ultrasonic wave, the second transducer generates an electrical signal, which is used to again trigger the propagation of an ultrasonic wave from the first transducer. This process is repeated and the repetition frequency of the output signals from the first transducer then corresponds to the transit time of ultrasonic waves passing from the first transducer to the second transducer. When the same technique is used will respect to ultrasonic waves propagated in the opposite direction, a pair of frequencies result, with the difference between the frequencies being a measure of the difference between the upstream propagation velocity of the ultrasonic wave and the downstream propagation velocity, hence a measure of the flow velocity of the fluid. The following U.S. Patents are examples of these prior art techniques, U.S. Pat. Nos. 2,746,291; 3,237,453; 3,869,915 and 3,812,098.
One problem with this technique is the problem of accurate measurement of a small difference in frequency. Other problems arise from reverberation effects in the conduit or fluid or ultrasonic transducers themselves rendering it still more difficult to obtain a precise measure of the difference in frequencies. Additionally, the presence of substantial material, such as buffer rods, between the transducers and the fluid leads to inaccuracy in correcting the frequency difference to fluid velocity.